(a) Field of the Invention
The present invention relates to a method and apparatus for a random access in a communications system, and more particularly, to a method and apparatus for a random access in a communications system operating a multi-beam.
(b) Description of the Related Art
In a mobile communications system, a random access procedure is performed so that uplink synchronization and uplink resource are allocated to a terminal in order for the terminal to perform an initial access and a connection setting with a base station. Besides, the random access procedure is also used for a scheduling request, a recovery of radio link failure, and the like. In LTE/LTE-A, the random access procedure is initiated by sending a specific preamble to a resource allocated for a given random access by the terminal.
In LTE/LTE-A, a Zadoff-Chu (ZC) sequence having a length of 839 or 139 is used as a preamble for the random access, and 64 different ZC sequences are allocated to one cell for random access. The 64 ZC sequences allocated to one cell are classified into a contention based random access sequence group and a non-contention based random access sequence group, and the contention based random access sequence group is classified into two groups, which are a group A and a group B. Depending on an amount of MAC data to be transmitted to the base station by the terminal in the random access procedure and a state of a channel, the sequence in the group A or the group B is selectively used. In a general random access, the terminal selects one among the sequences in the contention based random access sequence group and uses the selected sequence for the random access.
Meanwhile, depending on characteristics of the ZC sequence, a specific ZC sequence and a sequence obtained by cyclic shifting the specific ZC sequence are perpendicular to each other. Thus, in LTE/LTE-A, the 64 ZC sequences to be used for the random access for each cell may be configured of a root ZC sequence and the ZC sequence generated by cyclic shifting the root ZC sequence. Thereby, interference between the preambles may be minimized in the random access procedure within the cell, and inter-cell interference may also be decreased. A time in which the terminal may transmit the preambles in a specific cell, and a location of a frequency resource are informed to the terminal through system information by the base station, and the terminal arbitrarily selects one among the preambles usable in an allocated resource and transmits the selected preamble.
A general procedure of the random access will be described in more detail. The base station informs the system information including preamble information, maximum transmission power information, and the like for the random access to all of terminals in the cell. The terminal receiving the system information selects an arbitrary preamble suitable for a situation of the terminal among the preambles usable for the random access and transmits the selected preamble to the base station using the allocated time and frequency resource. The base station attempts to detect the preamble from the time and frequency resource allocated to the random access, and if the preamble is received, the base station transmits a random access response for the received preamble to the terminal. In this case, when several terminals in the cell use the same preamble in the same time and the same frequency resource, the base station does not distinguish a plurality of terminals and a contention resolution procedure should be performed later. The random access response includes timing advance (TA) information for matching the uplink synchronization, index information of the detected preamble, and uplink resource allocation information to be used for a future random access procedure. The random access response is transmitted via PDSCH, and PDCCH for a corresponding PDSCH is scrambled with RA-RNTI. Here, RA-RNTI is generated as expressed in Equation 1.RA-RNTI=1+t_id+10*f_id  [Equation 1]
Here, t_id, which is an integer between 0 and 10, denotes an index for a first subframe of a specific physical random access channel (PRACH), and f_id, which is an integer between 0 and 6, denotes a frequency index for the specific PRACH.
The terminal receiving PDCCH scrambled with RA-RNTI corresponding to the resource transmitted by the terminal performs a network entrance procedure such as transferring an RRC connection request message using the uplink resource allocation information allocated to the random access response and is switched to a state in which data may be transmitted and received.
Meanwhile, since an attenuation of a channel seriously occurs in a mobile communications system based on a millimeter wave of 30 GHz or more, the mobile communications system should use a directional beam in order to compensate for the attenuation. Since a channel of a millimeter wave band has strong linearity of a radio wave, one directional beam may cover only a specific area within the cell. Thus, in order to completely cover the cell by one base station, a plurality of directional beams should be simultaneously operated. A receiving terminal of each directional beam may include an RF module and a base band modem. In this case, the signal received by each beam may be independently demodulated for each beam. Alternatively, one RF module and one base band modem may be provided to several beams. In this case, data may be independently demodulated for the reception signal of each beam and the reception signals of the several beams may also be collected to demodulate data at the same time.
As such, in a communications system in which the base station uses the plurality of beams, the base station may receive the signal transmitted from one terminal through the several beams depending on channel characteristics between the base station and the terminal. Thus, in the case in which the random access procedure such as an existing LTE/LTE-A is used in the communications system using the plurality of beams, the preambles transmitted from the terminal may be detected from the plurality of beams of the base station, and in the case in which a preamble detection is each attempted for each beam, a lot of time and computation may be spent for the preamble detection.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.